by Roberto Acosta, Yoav Rosen and,Ra'ananAriav, Phibro Aqua, Phibro Animal Health Corporation, Ecuador and Phibro Aqua, Phibro Animal Health Corporation, Israel

 

Seafood production has been steadily increasing sincethe late 80's due to the contribution of the aquaculture sector. Moreover, the demand for aquaculture species such as fish, shellfish, and crustaceans is expected to continue to grow in the near future.

In order to keep up with this predicted demand for seafood, the aquaculture sector needs to develop innovativeand environmentally friendly solutions that improve cultured species survival and growth,while simultaneously reducing production costs.

Aquaculture feed plays a crucial role on the performance and health of the culture species, and accounts for a large portion of production costs. It is, therefore, critical to continue to develop feeds that increase feed efficiency, improve animal health, and produce less waste.

Feed additives have played an important role to improve aquafeeds. Saponins, por example, are an important and established feed additive in seafood aquaculture. In this article,the beneficial effects of saponins as feed additives are explored, with especial emphasis on its use for the culture of the Pacific white shrimp (Penaeus vannamei), which represents about half of the crustacean aquaculture production (53%).

The effect of saponins to improve nutrient absorption, digestive capacity, and growth performance is discussed together with the positive effects of saponins in shrimp immune system and its resistance to pathogens. Moreover, the use of saponins to reduce the nutrient load of shrimp aquaculture effluents is also reviewed.

 

Aquaculture: Global status and trends

Aquaculture production is the fastest growing food production sector and is responsible for a major increase in seafood supply for human consumption, especially since the late 80's when global seafood capture production stagnated. According to the latest report published in 2018 by the Food and Agriculture Organisation of the United Nations, aquaculture represented 47 percent of the total global seafood production in 2016 with 80 million tonnes registered in FAO records.

Moreover, 30.1 million tonnes of aquatic plants and 37,900 tonnes of non-food products were cultured. The total estimated first sale value of global aquaculture production in 2016 sums up to US $243.5(FAO, 2018) and 19,271 thousand farmers account for global employment statistics.

Among the main groups of seafood production, crustaceans comprise 9.8 percent of the world aquaculture production, with a total of 64 cultured species. Other groups include finfish and shellfish representing 67.6 percent and 21.4 percent of cultured species, respectively.

The main species of cultured crustaceans are prawns, crayfish, and crabs, the most commonly produced species being the Pacific white shrimp (Penaeus vannamei) that represents more than half of the crustacean production (53%). The remaining top five produced crustacean species are the red swamp crayfish (Procambarusclarkii, 12%), the Chinese mitten crab (Eriocheirsinensis, 10%), the giant tiger prawn (Penaeus monodon, 9%), the oriental river prawn (Macrobrachiumnipponense, 4%) and the giant river prawn (Macrobrachiumrosenbergii, 3%). The production of marine shrimps clearly dominates crustacean aquaculture, mainly taking place in Asian and Latin American countries, including China, Vietnam, Indonesia, India, Ecuador and Thailand.

Currently, the global demand for seafood continues to grow significantly due to a variety of social and economic factors, namely, population growth, rising incomes, and improved supply chain distribution from producers to the final consumers. Consumption per capita has increased on average by 1.5 percent per year since 1961, rising from 9.0 kg in 1961 to an estimated 20.5 kg in 2017.

The seafood trade for human consumption and non-food purposes (eg animal feed, pharmaceutical, nutraceutical, cosmetics, among others) plays a major role in economic development, especially in developing countries, boosting a wide range of industries and activities such as resource management, infrastructure and equipment construction, research and technology, and food processing.

In order to keep up with the demand and meet the United Nations' 2030 Agenda for Sustainable Development, the aquaculture industry has the opportunity to improve its sustainability in light of a changing world.

As such, future trends for aquaculture production include (i) a reduction in the use of antibiotics in order to mitigate the expansion of resistant microbial strains, (ii) the proliferation of Integrated Multi-trophic Aquaculture (IMTA) systems that reduce environmental degradation and excess excretion of nitrogen and phosphorus to the environment, (iii) the improvement of feed formulations, as the farming of fed species continues to rise in relation to the farming of unfed species, (iv) and the evolution of aquaculture systems and equipment, improving productivity and reducing the potential environmental impacts.

Aquaculture feed often accounts for half of the total production costs. It is, therefore, critical to develop innovative feed formulations and new ingredients that improve the nutritional quality of seafood products, particularly by increasing feed efficiency and metabolic assimilation of nutrients, as well reducing feed waste by improving feed distribution systems and promoting circular economy endeavors.

These are, indeed, among the top priorities in the aquaculture industry. Feeding efficiency and better nutritional profiles can be achieved through the inclusion of feed additives in aquafeeds, contributing to increased production rates and a reduction in the occurrence of infectious diseases such as white spot syndrome and early mortality syndrome, which are still a major threat to shrimp aquaculture as they lead to mass mortality events and great economic losses.

Ultimately, feed additives that improve digestive performance and reduce the occurrence of pests and diseases contribute to decreasing animal stress and, therefore, improving animal welfare in aquaculture facilities. In the next section, we highlight the importance of feed additives in aquaculture.

 

The importance of food additives in aquaculture: The specific case of saponins

The intensification of aquaculture production is not without challenges, as it can lead to sub-optimal culture conditions leading to animal stress and poor water quality, which limit the growth performance and welfare of cultured animals.

Thus, feed additives are commonly used in aquaculture production as a means to reduce stress, boost reproductive efficiency, improve individual health status and immune response, and enhance growth performance by improving nutrient retention, digestion, and assimilation.

Feed additives that promote the welfare of animals are especially relevant for shrimp aquaculture as shrimps lack an adaptive immune system and solely rely on innate immunity to fight pathogens and diseases. A few examples of feed additives include glucans, vitamin C, probiotics, prebiotics, organic acids, nucleotides, carotenoids, bioactive fatty acids, and plant-derived supplements.

Currently, there is a growing market to incorporate natural compounds in aquafeeds due to the fact that final consumers prefer chemical-free food and products, while also demanding environmentally friendly production processes.

Additionally, the use of synthetic substances, such as antibiotics and hormones, is no longer accepted in several countries. Indeed, the use of antibiotics as growth-promoters in aquaculture was banned in 2006 by the European Union (Regulation 1831/2003/EC). Consequently, natural ingredients, such as plant-derived feedstuffs, are now commonly used in aquaculture as supplements and also as fish meal substitutes.

Nevertheless, it is important to highlight that plant feedstuffs must meet specific nutritional and non-nutritional characteristics for inclusion in aquafeeds, such as low levels of fibre, high protein content, appropriate amino acid profile, high nutrient digestibility, as well as suitable palatability to maximise feed acceptance and intake.

Among the non-nutritional characteristics, availability, sustainability, price, easiness of processing, and storage and functionality (durability, expansion, water stability, oil absorption) are critical features to be evaluated.

Several plant and algal extracts from aloe vera, turmeric, cinnamon, propolis, Echinacea, garlic, green tea, cumin, ginger, soapbark tree, Mojave yucca, microalgae Naviculasp, Spirulina, and Ulva sp, among others, have already been tested and demonstrated for positive effects on survival rates, haematological and immune parameters, and improvement of growth performance of fish and shrimps.

Among such dietary supplements, saponins extracted from soapbark tree (Quillajasaponaria) and Mojave yucca (Yucca schidigera) have proven to be very promising ingredients for aquafeeds as natural growth promoters.

Saponins are classified as substances with multiple benefits, particularly in key parameters such as feed intake, nutrient digestibility, intestinal physiology, metabolism, growth, and health. These compounds are naturally occurring plant glycosides that have a steroid or triterpenoid structure and possess detergent properties.

However, at high concentrations, it is important to note that saponins can have deleterious effects on aquatic animals, such as depression of feed intake, inhibition of active uptake of nutrients, reduction of fertility and a decrease in protein digestibility.

Nevertheless, health-promoting effects, such as anti-carcinogenic, anti-microbial, cholesterol decreasing, immune modulating and anti-inflammatory activity, have been reported when saponins are used at lower concentrations and in properly balanced feed supplementation formula.

 

Beneficial effects of saponins in the nutrient absorption, digestive capacity and growth performance of aquaculture species

One of the major aquaculture production challenges is the development of feed formulations that replace scarce and costly ingredients, such as fishmeal and fish oil, and that improve feed conversation ratios (FCR) and growth performance.

Saponins are established candidates as feed supplements because they have been shown to increase the permeability of small intestinal mucosal cells, thus increasing nutrient absorption, particularly of macromolecules. Moreover, their detergent-like activity improves the digestibility of carbohydrates by reducing viscosity.

The stimulation of digestive enzyme activity like amylase, trypsin, alkaline protease, leucine aminopeptidase, alkaline phosphatase, and lipase has also been reported, along with a boost in respiratory enzymes like cytochrome c-oxidase. Such reports highlight the potential of saponins to increase the digestibility of nutrients, mainly proteins and carbohydrates, while simultaneously favoring anabolic processes by enhancing aerobic metabolism.

Several studies have demonstrated the capacity of saponins to increase the growth performance of cultured fish and shrimp. For example, common carp fed diets containing Quillaja saponins showed higher metabolic efficiency and average body weights (significantly increased by 37.5 to 73.2%), faster growth rates (specific growth rate increased 0.7 to 1.18% per day), and increased utilisation efficiency indexes, including food conversion efficiency, protein productive value, and protein gain while simultaneously maintaining routine metabolic rates.

Similar results were observed for Nile tilapia, revealing significantly higher energy retention and lipid conversion when supplemented with a saponin blend. Supplements based on three percent saponin blend, particularly from Quillajaand Yucca extracts, showed beneficial effects on weight and length gain, growth and survival rates, and better feed conversion ratios of Pacific white shrimp, leading to higher biomass yield per tank and an increase of 15-to-26 percent in total production.

Similarly, the use of a diet supplemented with saponin rich Quillaja extracts and vitamin C, led to an improvement of 14 percent in production and 15 percent increase in survival rates of Pacific white shrimp, as compared to the administration of control diets. Results from another scientific study also show that the FCR was improved by 23 percent in shrimps fed with the saponin-supplemented diet.

In summary, saponin-supplemented diets have important functional benefits for aquaculture species, particularly as growth promoters and gut health products driven by an enhanced feed utilisation efficiency.

Such results are thought to be due to a greater nutrient absorption brought on by higher intestine permeability and higher activity of digestive enzymes associated with saponin supplementation.

 

Beneficial effects of saponins in the immune system and resistance to pathogens

The spread of diseases is considered one of the most considerable problems in shrimp aquaculture. Diseases are often driven by poor water quality, high shrimp densities, nutritional imbalances, and a lack of proper biosecurity measurements associated with the trade of live infected animals among facilities.

Shrimp ponds are prone to the invasion of several pathogens including protozoa, fungi, and bacteria. However, the biggest threat comes from the spread of viral infections, which have caused great losses in several shrimp producing countries, including in Southeast Asia (Taiwan, China, Indonesia and India) and South America (Ecuador, Honduras and Mexico).

The top five most significant viruses affecting shrimp aquaculture are infectious hypodermal and hematopoietic necrosis virus (IHHNV), yellow head virus (YHV), Taura syndrome virus (TSV), white spot syndrome virus (WSSV), and infectious myonecrosis virus (IMNV).

Preventive measures are currently the best approach to reduce the spread of viral infections as there are no effective treatments for viral pandemic outbreaks in shrimp aquaculture facilities. As such, it is of the upmost importance to maintain optimal water quality, but also to enhance the immune response of shrimps.

Immunostimulant compounds, easily administered through feed, may be a solution to attenuate the problem of infectious diseases and consequently improve production yields. Immunostimulants are defined as compounds that 'enhance the innate or non-specific immune response by interacting directly with cells of the system, activating them'.

Several compounds have been administered as immunostimulants in aquaculture facilities to improve the health status of shrimps, including peptidoglycans, lipopolysaccharides, oligosaccharides, vitamins, nucleotides, antibacterial peptides, cytokines, probiotics, and herbal, plant and algae extracts.

Several modes of action of immunostimulants have been used in penaeid shrimp, such as: (i) enhancing the phagocytosis of pathogens through the activation of phagocytic cells in the hemolymph, (ii) increasing antibacterial and antiseptic properties of the hemolymph, (iii) activating signal transduction and the prophenoloxidase system.

More specifically, plant extracts have been reported to improve non-specific immune properties like leucocyte function, acting against a broad spectrum of pathogens. Nevertheless, several factors should be accounted for when administering immunostimulants as their efficiency depends on the timing of administration, dosage, and mode of action.

Saponins are among the plant extracts that have been shown to improve the immune response of shrimps and their resistance to pathogens. Immune modulation induced by saponins is apparently related to (i) the induction of cytokines like interleukins and interferons, (ii) the formation of complexes between saponins and antigens and their incorporation into cell or endosomal membranes, exposing antigens to cytosolic proteases (which would otherwise be exposed to digestive degradation), and (iii) the inhibition of non-specific processes such as inflammation and monocyte proliferation.

White shrimp contaminated with Vibrio alginolyticus and immersed in saponin solutions showed increased phagocytic activity and greater clearance efficiency, having higher survival rates, proving the immunomodulatory capacity of saponins.

Immune parameters, such as hyaline cells, total haemocyte count, specific α2-macroglobulin activity, respiratory burst, and antioxidant enzyme activity have been shown to increase with saponin supplementation.

Other reports similarly showed the enhancement of bacterial clearance in rainbow trout and the enhancement of chemotactic activity of yellowtail leucocytes. Moreover, another study carried out using the giant freshwater prawn also showed that saponins can modulate the immune system by increasing total haemocyte count and, ultimately, increasing disease resistance.

Besides the established immunostimulant effect, saponins have also been shown to be good vaccine adjuvants, to have antifungal properties, to reduce viral replication, and to induce detrimental effects on protozoa due to their detergent effect on cell membranes. Such properties are important to reduce the load of internal parasites in cultured shrimps.

 

The use of saponins to reduce the nutrient load of shrimp aquaculture effluents

Shrimp aquaculture is one of major sources of pollution in tropical and subtropical coastal areas due to discharges from culture ponds, creating excessive nitrogen loads from cultured animals' excretion as an end product of protein metabolism.

Ammonia, and its intermediate product nitrite, are highly toxic to aquatic animals, including fish and crustaceans and should be kept to a minimum in order to maintain animal welfare and, thus, maximise shrimp survival.

Water quality deterioration due to excess ammonia is a major issue in shrimp aquaculture and has been associated with collapses in production, mainly due to the rapid spread of diseases and physiological stress.

Moreover, nutrient excess may lead to the eutrophication of coastal ecosystems, causing mass mortality events. As an example, for every ton of cultured fish, 44-to-66kg of nitrogen and 7.2-to-10.5kg of phosphorus waste are produced. Several innovative and technological solutions have been proposed, in order to mitigate the pollution induced by pond effluents, such as IMTA, improved pond design, construction of buffer ponds, and bioreactors or bio-filters, in addition to reduction agents to treat effluent water and reduce water exchange rates.

However, the elevated costs of technology, poor planning, and lack of regulation may hamper the use of such innovative methods. Thus, a straightforward way to reduce the load of nutrients into the coastal environments might be to improve the nutrient composition of feed, given that the dietary requirements and welfare of the animals are still met.

From this point of view, the use of saponins as feed additives is of major interest as plant extracts that contain saponins and glycocomponents are able to bind to ammonia and mediate the conversion of ammonia to nitrite and to nitrate, the latter far less toxic form of nitrogen.

Moreover, HCO3- may react with ammonia to form urea in the presence of saponins. Yucca extracts have been successfully used in livestock husbandry to control ammonia accumulation in the facilities as well as to reduce odour.

According to this latter study, Yucca extracts can also be used in aquaculture facilities. Bioassays using fish and shrimp in both freshwater and saltwater systems have shown that Yucca and Quillaja extracts reduce ammonia when used as feed additives or liquid extracts for water treatment.

For example, a Yucca extract added at 6mg.L-1 every 15 days to fish and shrimp systems achieved a 58-60 percent reduction in ammonia as compared to a control system. In another study, the addition of Yucca extract at 430 mg.L-1 to 30 L tanks in a recirculating water system achieved an 82 percent reduction in ammonia. Still, the inhibition of ammonia leaching from feces and feed waste and, therefore, the reduction of ammonia levels in water was improved when Yucca or Quillaja extracts were used as feed supplement as compared to water treatment solutions.

 

Conclusion

Saponins are important and established feed additives in seafood aquaculture. Overall, and taking into consideration the positive digestive and growth performance effects particularly observed for Pacific white shrimp, there is overwhelming scientific evidence that saponins can notably contribute to boost shrimp aquaculture production and profitability.

Additionally, saponins have shown positive effects in the immune system of aquatic species and their resistance to pathogens. The integration of saponins in aquafeeds is, therefore, a relevant step to improve animal welfare, control infectious diseases, and further advance health management strategies in aquaculture production.

Lastly, the use of saponins in feed formulations can help fish and shrimp farmers in ammonia management strategies, thereby contributing to the establishment of environmentally friendly production processes in order to achieve the sustainable development goals enacted by the United Nations for 2030.

In summary, the recognised properties of saponins as growth prometers,immunostimulants, parasitic control, and ammonia reducing agents will certainly help seafood farmers to achieve not only higher production levels and profitability, but also establish effective health management strategies and eco-friendly production processes.

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